Natural gas from either production reservoirs or storage reservoirs typically contains water, as well as other species, which form solids during the liquefaction to produce liquefied natural gas (LNG). It is common practice for the natural gas to be subjected to a dehydration process prior to the liquefaction. Water is removed to prevent hydrate formation occurring in pipelines and heat exchangers upstream of the liquefaction vessel.
If water is not removed, solid hydrates may form in pipe work, heat exchangers and/or the liquefaction vessel. The hydrates are stable solids comprising water and natural gas having the outward appearance of ice, with the natural gas stored within the crystal lattice of the hydrate.
The formation of natural gas hydrates was historically seen as an undesirable result that should be avoided. However, processes have been developed to encourage natural gas hydrate formation such as International patent applications No. 01/00 755 and No. 01/12 758. In the first of these International patent applications, a method and apparatus is described whereby natural gas and water are combined in the presence of an agent adapted to reduce the natural gas water interfacial tension to encourage natural gas hydrate formation. In the second of these International patent applications, a production plant is described, including a convoluted flow path to cause mixing of water and natural gas as a first step prior to reducing the temperature to produce natural gas hydrate.
Methods of dehydrating natural gas feed streams include absorption of water in glycol or adsorption of the water using a solid such as hydrated aluminium oxide, silica gels, silica-alumina gels and molecular sieves.
Natural gas also typically contains sour species, such as hydrogen sulphide (H2S) and carbon dioxide (CO2). Such a natural gas is classified as “sour gas”. When the H2S and CO2 have been removed from the natural gas feed stream, the gas is then classified as “sweet”. The term “sour gas” is applied to natural gases including H2S because of the bad odour that is emitted even at low concentrations from an unsweetened gas. H2S is a contaminant of natural gas that must be removed to satisfy legal requirements, as H2S and its combustion products of sulphur dioxide and sulphur trioxide are also toxic. Furthermore, H2S is corrosive to most metals normally associated with gas pipelines so that processing and handling of a sour gas may lead to premature failure of such systems.
Gas sweetening processes typically include adsorption using solid adsorption processes or absorption using amine processes, molecular sieves, etc. Existing dehydration and gas sweetening processes are extremely complex and expensive.